If you are talking about the plumbing hooked to the septic system which is waste plumbing, unless it is leaking there is no real need to replace it.
If you are speaking about old galvanized supply plumbing the th fixtures, there is no need to remove it all. Disconnect at the lowest point to drain it, then disconnect the connections to the fixtures and as much as you can get to. To remove it all will require rather extensive removal of walls, floors or ceiling sections. Once it is disconnected and drained you can run new PX tubing as a replacement. Because PX is flexible you can feed it through walls and ceilings with minimal tear out.
Be sure you live your life, because you are a long time dead.-Scottish Proverb